Frequency doubling is often used in electronic circuits. Where a single or narrow range of frequencies is encountered, resonant harmonic circuits are common. However, where a large range of frequencies is encountered, more complicated circuits must be employed. In one common application an electronic tachometer is used to count wheel or shaft revolutions. The rotor of an electromagnetic generator is attached to the shaft and the generator output frequency is proportional to shaft rotation rate. Typically the a-c signal is amplified, squared, rectified, and filtered to produce a d-c voltage proportional to speed. Clearly the filter must have a long time constant to do an adequate filtering job at the lower frequencies. Such filters have a slow response to changes in voltage. If the generator frequency is electronically doubled, the related filter can be designed to have doubled response speed. Accordingly, where response speed is marginal, this technique is widely used.
In prior art frequency doubling, one technique has been to square up the a-c input in an amplifier limiter, differentiate the resulting square wave, invert every other pulse and use the pulses to trigger a one shot multivibrator or equivalent circuit. Alternatively, the differentiated signals can be clipped and shaped, including inverting every other pulse, and used directly as a frequency doubled output. These schemes are either complicated and use much circuitry or are imprecise. Attempts to simplify the prior art circuits have also been complicated. Typically a matched pair of capacitors are alternately charged and discharged by alternate half input cycles to produce a pair of controlled width output pulses for each input cycle. It is important that each output pulse have the same width, hence the requirement for matched capacitor pairs. These are cumbersome as well as expensive.